As with all the bosses in the Monastery of Ascension, Legio Quartus attacks primarily with a combination of a basic magic attack and a lightning attack - the latter of which is focused in a 2x2 square (expanding to a 3x3 square and then to a 4x4 square over two game ticks) which deals high, magic-style damage to the player if they stand where the lightning falls. (The lightning attack is always affixed to the player's location, and thus continuously moving throughout the fight is imperative.)
Like all of the Legiones within the Monastery of Ascension, Legio Quartus has a special ability which activates as his life points are gradually depleted in 25% intervals (every 7,500 life points drained) until death - occurring a total of three times throughout the fight. His special ability teleports the player close to him and summons lightning at the arrival location of the teleport (giving the player a short time to move away in order to avoid considerably high damage). As with all other Legiones, his special ability's activation is signaled by an overhead chat message.
Legio Quartus is roughly Latin for "Fourth Legion". However Legio is a feminine noun, while Quartus is a masculine adjective; the correct form would be Legio Quarta. Legions were the largest units of the Roman army and there were 6 in a whole army, hence there are six legiones in the Monastery.